Manufacturing method of SOI substrate

ABSTRACT

There is provided a method of manufacturing an SOI substrate which is practicable even when a supporting substrate having a low allowable temperature limit is used. A separation layer is formed in a region at a certain depth from a surface of a semiconductor substrate, and a first heat treatment is conducted when a semiconductor layer on the separation layer is bonded to the supporting substrate and separated. A second heat treatment is conducted to the supporting substrate to which the semiconductor layer is bonded. The second heat treatment is conducted at a temperature which is equal to or higher than the temperature of the first heat treatment and does not exceed a strain point of the supporting substrate. When the first heat treatment and the second heat treatment are conducted at the same temperature, a treatment time of the second heat treatment may be set to be longer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a substrate having an SOI structure,which is formed by bonding a semiconductor layer formed from a thinslice of a crystalline semiconductor substrate to a different kind ofsubstrate. In particular, the present invention relates to a bonding SOItechnique, and also relates to a manufacturing method of an SOIsubstrate in which a single-crystalline or polycrystalline semiconductorlayer is bonded to a substrate having an insulating surface such as aglass substrate. Further, the present invention relates to a displaydevice or a semiconductor device using such a substrate having an SOIstructure.

2. Description of the Related Art

A semiconductor substrate called a silicon-on-insulator (SOI substrate)that has a thin single-crystalline semiconductor layer on an insulatinglayer has been developed instead of a silicon wafer that is manufacturedby thinly slicing an ingot of a single-crystalline semiconductor, andthe SOI substrate is spreading as a substrate in manufacturing amicroprocessor or the like. This is because an integrated circuit usingan SOI substrate draws attention as an integrated circuit in whichparasitic capacitance between drains of transistors and a substrate canbe reduced, performance of the semiconductor integrated circuit can beimproved, and low power consumption is achieved.

As a method for manufacturing SOI substrates, a hydrogen ionimplantation separation method is known (for example, see PatentDocument 1: U.S. Pat. No. 6,372,609). The hydrogen ion implantationseparation method is a method in which hydrogen ions are implanted intoa silicon wafer to form a microbubble layer at a predetermined depthfrom the surface, and a thin silicon layer (SOI layer) is bonded toanother silicon wafer using the microbubble layer as a cleavage plane.In addition to the heat treatment for separation of an SOI layer, it isnecessary to perform heat treatment in an oxidizing atmosphere to forman oxide film on the SOI layer, remove the oxide film, and perform heattreatment at 1000° C. to 1300° C. in a reducing atmosphere to increasebonding strength.

On the other hand, forming an SOI layer on an insulating substrate suchas glass is also attempted. As an example of SOI substrates in which SOIlayers are formed on glass substrates, an SOI substrate in which a thinsingle-crystalline silicon layer is formed over a glass substrate havinga coating film by a hydrogen ion implantation separation method is known(see Patent Document 2: U.S. Pat. No. 7,119,365). In this case also, thethin silicon layer (SOI layer) is formed over the glass substrate insuch a way that a microbubble layer is formed at a predetermined depthfrom the surface by implantation of hydrogen ions to asingle-crystalline silicon wafer, the glass substrate and thesingle-crystalline silicon wafer are bonded, and the silicon wafer isseparated using the microbubble layer as a cleavage plane.

SUMMARY OF THE INVENTION

In order to separate a single-crystalline silicon layer from a siliconwafer by a hydrogen ion implantation separation method, heat treatmentat a high temperature equal to or higher than 600° C. has been required.However, in the case of forming an SOI substrate by bondingsingle-crystalline silicon to a glass substrate that is used in a liquidcrystal panel or the like for cost reduction of the substrate, there hasbeen a problem in that such heat treatment at a high temperature causeswarping of the glass substrate. When the glass substrate warps,reduction in bonding strength to the single-crystalline silicon layer isa concern. In addition, there is also a problem in that strain stress isapplied to the single-crystalline silicon layer and characteristics ofthe transistor are adversely affected. In other words, with aconventional technique, even when a single-crystalline silicon layer isprovided over a glass substrate and a transistor is manufactured usingthe single-crystalline silicon layer, adequate characteristics cannot beobtained.

In consideration of such problems, it is an object to provide an SOIsubstrate including a crystalline semiconductor layer which ispracticable even when a substrate having a low allowable temperaturelimit such as a glass substrate is used. Further, it is another objectto provide a semiconductor device using such an SOI substrate.

The gist of the invention is to bond a single-crystalline semiconductorto a supporting substrate having an insulating surface at a temperatureequal to or lower than a strain point of the supporting substrate. Thissingle-crystalline semiconductor layer is formed through a plurality ofheat treatment steps.

A separation layer is formed in a region at a certain depth from asurface of a single-crystalline or polycrystalline semiconductorsubstrate, and first heat treatment is conducted when a semiconductorlayer on the separation layer is bonded to a supporting substrate andseparated. Then, second heat treatment is conducted to the supportingsubstrate to which the semiconductor layer is bonded. The first heattreatment and the second heat treatment are preferably conducted atdifferent temperatures. In this case, the second heat treatment ispreferably conducted at a temperature which is higher than thetemperature of the first heat treatment and does not exceed a strainpoint of the substrate to which the semiconductor layer is bonded.Alternatively, when the first heat treatment and the second heattreatment are conducted at the same temperature, the treatment time ofthe second heat treatment may be set to be longer than that of the firstheat treatment.

The separation layer provided at a predetermined depth from the surfaceof the single-crystalline or polycrystalline semiconductor substrate isformed by introducing accelerated ions from the surface of thesemiconductor substrate. The ions introduced preferably include ions ofone kind or ions of plural kinds of the same type of atom which havedifferent masses. For example, a predetermined gas is made into plasmato generate a plurality of ion species, and without mass separation, theion species are accelerated by electric field and introduced to asingle-crystalline or polycrystalline semiconductor substrate.Typically, hydrogen ions are selected, and H⁺, H₂ ⁺, and H₃ ⁺ ions areemployed as the ions of plural kinds of the same type of atom which havedifferent masses. In this case, the proportion of H₃ ⁺ ions ispreferably made higher than those of the other ion species.

In fixing a semiconductor layer which is thinly separated from asingle-crystalline or polycrystalline semiconductor substrate to asupporting substrate, a layer which has a smooth surface and forms ahydrophilic surface is provided for one or both of surfaces which form abond, as a bonding surface. As such a layer, a silicon oxide layer istypically employed. This silicon oxide film is preferably formed bythermal oxidation, chemical reaction, or chemical vapor deposition.

In bonding a crystalline semiconductor layer which is separated from asingle-crystalline or polycrystalline semiconductor substrate to asupporting substrate, a substrate having an SOI structure superior incrystallinity can be obtained by conducting plural heat treatments. Evenwhen the supporting substrate and the crystalline semiconductor layer,which are bonded to each other, have different thermal characteristicssuch as strain points and coefficients of thermal expansion, an SOIsubstrate superior in crystallinity with relaxed distortion can beobtained by conducting heat treatment by which a bond is formed andlater heat treatment at different temperatures and/or differenttreatment times.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are cross-sectional views of structures of a substratehaving an SOI structure;

FIGS. 2A and 2B are cross-sectional views of structures of a substratehaving an SOI structure;

FIGS. 3A to 3C are cross-sectional views of a manufacturing method of asubstrate having an SOI structure;

FIGS. 4A and 4B are cross-sectional views of a manufacturing method of asubstrate having an SOI structure;

FIGS. 5A to 5C are cross-sectional views of a manufacturing method of asubstrate having an SOI structure;

FIGS. 6A to 6D are cross-sectional views of a manufacturing method of asemiconductor device using a substrate having an SOI structure;

FIG. 7 is a cross-sectional view of a manufacturing method of asemiconductor device using a substrate having an SOI structure;

FIG. 8 is a block diagram of a structure of a microprocessor which isobtained using a substrate having an SOI structure;

FIG. 9 is a block diagram of a structure of an RFCPU which is obtainedusing a substrate having an SOI structure;

FIG. 10 is a plane view showing an example in the case of bondingsingle-crystalline semiconductor layers to a mother glass formanufacturing a display panel;

FIGS. 11A and 11B show an example of a liquid crystal display panel inwhich a pixel transistor is formed using a single-crystallinesemiconductor layer;

FIGS. 12A and 12B show an example of an electroluminescent display panelin which a pixel transistor is formed using a single-crystallinesemiconductor layer;

FIGS. 13A to 13C are cross-sectional views of a manufacturing method ofa substrate having an SOI structure according to Embodiment 1;

FIGS. 14A and 14B are cross-sectional views of a manufacturing method ofa substrate having an SOI structure according to Embodiment 1;

FIG. 15 is a graph showing Raman spectra of single-crystallinesemiconductor layers of substrates each having an SOI structureaccording to Embodiment 1;

FIGS. 16A to 16C are cross-sectional views of a manufacturing method ofa substrate having an SOI structure;

FIG. 17 is an energy diagram of hydrogen ion species;

FIG. 18 is a diagram showing the results of ion mass spectrometry;

FIG. 19 is a diagram showing the results of ion mass spectrometry;

FIG. 20 is a diagram showing the profile (measured values and calculatedvalues) of hydrogen in the depth direction when the accelerating voltageis 80 kV;

FIG. 21 is a diagram showing the profile (measured values, calculatedvalues, and fitting functions) of hydrogen in the depth direction whenthe accelerating voltage is 80 kV;

FIG. 22 is a diagram showing the profile (measured values, calculatedvalues, and fitting functions) of hydrogen in the depth direction whenthe accelerating voltage is 60 kV;

FIG. 23 is a diagram showing the profile (measured values, calculatedvalues, and fitting functions) of hydrogen in the depth direction whenthe accelerating voltage is 40 kV; and

FIG. 24 is a list of ratios of fitting parameters (hydrogen atom ratiosand hydrogen ion species ratios).

DETAILED DESCRIPTION OF THE INVENTION

Embodiment mode of the present invention will be described withreference to the drawings. It is easily understood by those skilled inthe art that various changes may be made in forms and details withoutdeparting from the spirit and the scope of the invention. Therefore, thepresent invention should not be interpreted as being limited to thedescription of the embodiment mode below. In structures of the presentinvention described below, the same reference numerals are commonlygiven to the same components or components having similar functionsthroughout the drawings.

FIGS. 1A and 1B show substrates each having an SOI structure accordingto this embodiment mode. In FIG. 1A, a supporting substrate 100 has aninsulating property or an insulating surface, and glass substrates usedfor electronics industry (also called a “non-alkali glass substrate”)such as an aluminosilicate glass substrate, an aluminoborosilicate glasssubstrate, or a barium borosilicate glass substrate can be used. Inother words, a glass substrate having a coefficient of thermal expansionof from 25×10⁻⁷/° C. to 50×10⁻⁷/° C. (preferably, from 30×10⁻⁷/° C. to40×10⁻⁷/° C.) and a strain point of from 580° C. to 680° C. (preferably,from 600° C. to 680° C.) can be used. Alternatively, a quartz substrate,a ceramic substrate, a metal substrate having a surface coated with aninsulating film, or the like can be used.

Single-crystalline silicon is typically used for a single-crystallinesemiconductor layer 101. Alternatively, silicon or germanium which canbe separated from a single-crystalline semiconductor substrate or apolycrystalline semiconductor substrate by a hydrogen ion implantationseparation method can be used, or a single-crystalline semiconductor ora polycrystalline semiconductor of a compound semiconductor such asgallium arsenide or indium phosphide can be used.

Between the supporting substrate 100 and the single-crystallinesemiconductor layer 101, a bonding layer 102 which has a smooth surfaceand forms a hydrophilic surface is provided. This bonding layer 102 is alayer which has a smooth surface and has a hydrophilic surface. As alayer which can form such a surface, an insulating layer formed bychemical reaction is preferable. For example, an oxide film which isformed by thermal reaction or chemical reaction is appropriate for thebonding layer 102. This is mainly because the smoothness of the surfacecan be secured when using a film formed by chemical reaction. Thebonding layer 102 which has a smooth surface and forms a hydrophilicsurface is provided with a thickness of from 0.2 nm to 500 nm. With thisthickness, it is possible to smooth surface roughness of a surface onwhich a film is to be formed and also to ensure smoothness of a surfaceof the film.

When the single-crystalline semiconductor layer 101 is formed usingsilicon, silicon oxide formed by heat treatment in an oxidizingatmosphere, silicon oxide which grows due to reaction of oxygenradicals, a chemical oxide formed using an oxidizing chemical solution,or the like can be formed as the bonding layer 102. In the case of usinga chemical oxide as the bonding layer 102, the thickness of the chemicaloxide may be from 0.2 nm to 1 nm. Preferably, silicon oxide deposited bychemical vapor deposition can be used as the bonding layer 102. In thiscase, a silicon oxide film formed by chemical vapor deposition using anorganic silane gas is preferable. As the organic silane gas, asilicon-containing compound such as tetraethoxysilane (TEOS: chemicalformula, Si(OC₂H₅)₄), tetramethylsilane (TMS: chemical formula,Si(CH₃)₄), tetramethylcyclotetrasiloxane (TMCTS),octamethylcyclotetrasiloxane (OMCTS), hexamethyldisilazane (HMDS),triethoxysilane (SiH(OC₂H₅)₃), or trisdimethylaminosilane(SiH(N(CH₃)₂)₃) can be used.

The bonding layer 102 is provided on the single-crystallinesemiconductor layer 101 side and is made in close contact with a surfaceof the supporting substrate 100. In this way, bonding can be performedeven at a room temperature. To form a stronger bond, the supportingsubstrate 100 and the single-crystalline semiconductor layer 101 may bepressed. For bonding between the supporting substrate 100 and thebonding layer 102 which are formed using different materials, surfacesthereof are cleaned. When the cleaned surfaces of the supportingsubstrate 100 and the bonding layer 102 are made in close contact witheach other, a bond is formed by attractive force between the surfaces.Further, a more preferred mode can be obtained by adding treatment inwhich many hydrophilic groups are attached to the surface. For example,it is preferable that the surface of the supporting substrate 100 besubjected to oxygen plasma treatment or ozone treatment to have ahydrophilic property. In the case of adding the treatment by which thesurface is made to have a hydrophilic property in this manner, hydroxylgroups on the surface act to form a bond due to hydrogen bonding.Further, the bond between the cleaned surfaces, which is formed bymaking the surfaces closely contact with each other, can be strengthenedby being heated at a temperature equal to or higher than a roomtemperature.

As the treatment for bonding the supporting substrate 100 and thebonding layer 102 which are formed using different materials, thesurfaces which form a bond may be irradiated with an ion beam using aninert gas such as argon so as to be cleaned. By the ion beamirradiation, dangling bonds are exposed on the surfaces of thesupporting substrate 100 and the bonding layer 102, and the surfacesbecome very active. In this way, when activated surfaces are made inclose contact with each other, a bond can be formed even at a lowtemperature. In the method of forming a bond by activating surfaces,since it is required to keep the surfaces in a highly clean state, themethod is preferably carried out in vacuum.

The single-crystalline semiconductor layer 101 is formed from a thinslice of a crystalline semiconductor substrate. For example, thesingle-crystalline semiconductor layer 101 can be formed in such amanner that hydrogen ions or fluorine ions are introduced to apredetermined depth of a single-crystalline semiconductor substrate,heat treatment is then conducted, and the single-crystalline siliconlayer, which is a surface layer, is separated. Alternatively, a methodin which single-crystalline silicon is epitaxially grown on poroussilicon and the porous silicon layer is cleaved by water jetting so asto be separated, may also be applied. The thickness of thesingle-crystalline semiconductor layer 101 is from 5 nm to 500 nm, andpreferably from 10 nm to 200 nm.

FIG. 1B shows a structure in which a barrier layer 103 and the bondinglayer 102 are provided for the supporting substrate 100. By provision ofthe barrier layer 103, the single-crystalline semiconductor layer 101can be prevented from being contaminated by diffusion of a movable ionimpurity such as an alkali metal or an alkaline earth metal from theglass substrate used as the supporting substrate 100. The bonding layer102 is preferably provided over the barrier layer 103. By providing theplurality of layers having different functions, which are the barrierlayer 103 for preventing diffusion of impurities and the bonding layer102 for securing the bonding strength, over the supporting substrate100, the supporting substrate can be selected from a wider range. Thebonding layer 102 is preferably provided on the single-crystallinesemiconductor layer 101 side as well. In other words, in bonding thesingle-crystalline semiconductor layer 101 to the supporting substrate100, the bonding layer 102 is preferably provided for one or both of thesurfaces which form a bond; accordingly, bonding strength can beheightened.

FIG. 2A shows a structure in which an insulating layer 104 is providedbetween the single-crystalline semiconductor layer 101 and the bondinglayer 102. The insulating layer 104 is preferably an insulating layercontaining nitrogen. For example, the insulating layer 104 can be formedby stacking one or more films selected from a silicon nitride film, asilicon nitride oxide film, or a silicon oxynitride film. For example,the insulating layer 104 can be formed by stacking a silicon oxynitridefilm and a silicon nitride oxide film from the single-crystallinesemiconductor layer 101 side. The bonding layer 102 has a function offorming a bond with the supporting substrate 100, whereas the insulatinglayer 104 prevents the single-crystalline semiconductor layer 101 frombeing contaminated by an impurity.

Note that a silicon oxynitride film means a film that contains moreoxygen than nitrogen and, in the case where measurements are performedusing Rutherford backscattering spectrometry (RBS) and hydrogen forwardscattering (HFS), includes oxygen, nitrogen, silicon, and hydrogen atconcentrations ranging from 50 at. % to 70 at. %, 0.5 at. % to 15 at. %,25 at. % to 35 at. %, and 0.1 at. % to 10 at. %, respectively. Further,a silicon nitride oxide film means a film that contains more nitrogenthan oxygen and, in the case where measurements are performed using RBSand HFS, includes oxygen, nitrogen, silicon, and hydrogen atconcentrations ranging from 5 at. % to 30 at. %, 20 at. % to 55 at. %,25 at. % to 35 at. %, and 10 at. % to 30 at. %, respectively. Note thatpercentages of nitrogen, oxygen, silicon, and hydrogen fall within theranges given above, where the total number of atoms contained in thesilicon oxynitride film or the silicon nitride oxide film is defined as100 at. %.

FIG. 2B shows a structure in which the bonding layer 102 is providedover the supporting substrate 100. The barrier layer 103 is preferablyprovided between the supporting substrate 100 and the bonding layer 102in order to prevent the single-crystalline semiconductor layer 101 frombeing contaminated by diffusion of a movable ion impurity such as analkali metal or an alkaline earth metal from the glass substrate used asthe supporting substrate 100. A silicon oxide layer 105 that is formedby direct oxidation is formed on the single-crystalline semiconductorlayer 101. This silicon oxide layer 105 forms a bond with the bondinglayer 102 and fixes the single-crystalline semiconductor layer 101 onthe supporting substrate 100. The silicon oxide layer 105 is preferablyformed by thermal oxidation.

A manufacturing method of such a substrate having an SOI structure isdescribed with reference to FIGS. 3A to 4B.

A separation layer 107 is formed by introducing ions accelerated byelectric field from a cleaned surface of a semiconductor substrate 106to a predetermined depth (see FIG. 3A). The depth of the separationlayer 107 formed in the semiconductor substrate 106 is controlled by theacceleration energy of ions and the incident angle of the ions. Theseparation layer 107 is formed in a region at a depth close to theaverage depth of introduced ions from the surface of the semiconductorsubstrate 106. For example, the thickness of the single-crystallinesemiconductor layer is from 5 nm to 500 nm, and preferably from 10 nm to200 nm, and accelerating voltage in introduction of ions is determinedin consideration of the thickness. The introduction of the ions ispreferably conducted with an ion doping apparatus. In other words, adoping method in which plural ion species generated by making a sourcegas into plasma are introduced without mass separation, is used. In thisembodiment mode, ions of one kind or ions of plural kinds of the sametype of atom which have different masses are preferably introduced. Atthe ion doping, the accelerating voltage may be from 10 kV to 100 kV,and preferably from 30 kV to 80 kV; the dose may be from 1×10¹⁶ ions/cm²to 4×10¹⁶ ions/cm²; and the beam current density may be equal to orgreater than 2 μA/cm², preferably equal to or greater than 5 μA/cm², andmore preferably equal to or greater than 10 μA/cm². By the introductionof ions, defects generated in the semiconductor layer can be reduced.

In the case of introducing hydrogen ions, it is preferable that H⁺, H₂⁺, and H₃ ⁺ ions are contained and the proportion of H₃ ⁺ ions is madehigher, because the number of hydrogen atoms that can be introduced to asemiconductor substrate in unit time is increased and thus the time forintroducing ions can be shortened. Accordingly, the region of theseparation layer 107 formed in the semiconductor substrate 106 cancontain hydrogen at a concentration equal to or higher than 1×10²⁰ atoms/cm³ (preferably, 5×10²⁰ atoms/cm³). When a region in which hydrogen isintroduced at a high concentration is locally formed in thesemiconductor substrate 106, the crystal structure is disordered andmicrocavities are formed, so that the separation layer 107 can have aporous structure. In this case, the volume of the microcavities formedin the separation layer 107 is changed by heat treatment at a relativelylow temperature, and cleavage is formed along the separation layer;accordingly, a thin single-crystalline semiconductor layer can beformed.

Here, H⁺, H₂ ⁺, and H₃ ⁺ ions formed from a hydrogen plasma will bedescribed. Reaction processes (formation processes, destructionprocesses) of the hydrogen ion species can be explained with reactionequations shown below.

e+H→e+H⁺ +e  (1)

e+H₂ →e+H₂ ⁺ +e  (2)

e+H₂ →e+(H₂)*→e+H+H  (3)

e+H₂ ⁺ →e+(H₂ ⁺)*→e+H⁺+H  (4)

H₂ ⁺+H₂→H₃ ⁺+H  (5)

H₂ ⁺+H₂→H⁺+H+H₂  (6)

e+H₃ +→e+H⁺+H+H  (7)

e+H₃ ⁺H₂+H  (8)

e+H₃ ⁺→H+H+H  (9)

FIG. 17 is an energy diagram which schematically shows some of the abovereactions. Note that the energy diagram shown in FIG. 17 is merely aschematic diagram and does not depict the relationships of energies ofthe reactions exactly.

(H₃ ⁺ Formation Process)

As shown above, H₃ ⁺ is mainly produced through the reaction processthat is represented by the reaction equation (5). On the other hand, asa reaction that competes with the reaction equation (5), there is thereaction process represented by the reaction equation (6). For theamount of H₃ ⁺ to increase, at the least, it is necessary that thereaction of the reaction equation (5) occur more often than the reactionof the reaction equation (6) (note that, because there are also otherreactions, (7), (8), and (9), through which the amount of H₃ ⁺ isdecreased, the amount of H₃ ⁺ is not necessarily increased even if thereaction of the reaction equation (5) occurs more often than thereaction of the reaction equation (6)). In contrast, when the reactionof the reaction equation (5) occurs less often than the reaction of thereaction equation (6), the proportion of H₃ ⁺ in a plasma is decreased.

The amount of increase in the product on the right-hand side (rightmostside) of each reaction equation given above depends on the density of asource material on the left-hand side (leftmost side) of the reactionequation, the rate coefficient of the reaction, and the like. Here, itis experimentally confirmed that, when the kinetic energy of H₂ ⁺ islower than about 11 eV, the reaction of the reaction equation (5) is themain reaction (that is, the rate coefficient of the reaction equation(5) is sufficiently higher than the rate coefficient of the reactionequation (6)) and that, when the kinetic energy of H₂ ⁺ is higher thanabout 11 eV, the reaction of the reaction equation (6) is the mainreaction.

A force is exerted on a charged particle by an electric field, and thecharged particle gains kinetic energy. The kinetic energy corresponds tothe amount of decrease in potential energy due to an electric field. Forexample, the amount of kinetic energy a given charged particle gainsbefore colliding with another particle is equal to the differencebetween a potential energy at a potential before the charged particlemoves and a potential energy at a potential before the collision. Thatis, in a situation where a charged particle can transfer a long distancein an electric field without colliding with another particle, thekinetic energy (or the average thereof) of the charged particle tends tobe higher than that in a situation where the charged particle cannot.Such a tendency toward an increase in kinetic energy of a chargedparticle can be shown in a situation where the mean free path of aparticle is long, that is, in a situation where pressure is low.

Even in a situation where the mean free path is short, the kineticenergy of a charged particle is high if the charged particle can gain ahigh amount of kinetic energy while traveling through the path. That is,it can be said that, even in the situation where the mean free path isshort, the kinetic energy of a charged particle is high if the potentialdifference is large.

This is applied to H₂ ⁺. Assuming that an electric field is present asin a plasma generation chamber, the kinetic energy of H₂ ⁺ is high in asituation where the pressure inside the chamber is low and the kineticenergy of H₂ ⁺ is low in a situation where the pressure inside thechamber is high. That is, because the reaction of the reaction equation(6) is the main reaction in the situation where the pressure inside thechamber is low, the amount of H₃ ⁺ tends to be decreased, and becausethe reaction of the reaction equation (5) is the main reaction in thesituation where the pressure inside the chamber is high, the amount ofH₃ ⁺ tends to be increased. In addition, in a situation where anelectric field in a plasma generation region is high, that is, in asituation where the potential difference between given two points islarge, the kinetic energy of H₂ ⁺ is high, and in the oppositesituation, the kinetic energy of H₂ ⁺ is low. That is, because thereaction of the reaction equation (6) is the main reaction in thesituation where the electric field is high, the amount of H₃ ⁺ tends tobe decreased, and because the reaction of the reaction equation (5) isthe main reaction in a situation where the electric field is low, theamount of H₃ ⁺ tends to be increased.

(Differences Depending on Ion Source)

Here, an example, in which the proportions of ion species (particularly,the proportion of H₃ ⁺) are different, is described. FIG. 18 is a graphshowing the results of mass spectrometry of ions that are generated froma 100% hydrogen gas (with the pressure of an ion source of 4.7×10⁻² Pa).Note that this mass spectrometry was performed by measurement of ionsthat were extracted from the ion source. The horizontal axis representsion mass. In the spectrum, the mass 1 peak, the mass 2 peak, and themass 3 peak correspond to H⁺, H₂ ⁺, and H₃ ⁺, respectively. The verticalaxis represents the intensity of the spectrum, which corresponds to thenumber of ions. In FIG. 18, the number of ions with different masses isexpressed as a relative proportion where the number of ions with a massof 3 is defined as 100. It can be seen from FIG. 18 that the ratiobetween ion species that are generated from the ion source, i.e., theratio between H⁺, H₂ ⁺, and H₃ ⁺, is about 1:1:8. Note that ions at sucha ratio can also be generated by an ion doping apparatus which has aplasma source portion (ion source) that generates a plasma, anextraction electrode that extracts an ion beam from the plasma, and thelike.

FIG. 19 is a graph showing the results of mass spectrometry of ions thatare generated from PH₃ when an ion source different from that for thecase of FIG. 18 is used and the pressure of the ion source is about3×10⁻³ Pa. The results of this mass spectrometry focus on the hydrogenion species. In addition, the mass spectrometry was performed bymeasurement of ions that were extracted from the ion source. As in FIG.18, the horizontal axis represents ion mass, and the mass 1 peak, themass 2 peak, and the mass 3 peak correspond to H⁺, H₂ ⁺, and H₃ ⁺,respectively. The vertical axis represents the intensity of a spectrumcorresponding to the number of ions. It can be seen from FIG. 19 thatthe ratio between ion species in a plasma, i.e., the ratio between H⁺,H₂ ⁺, and H₃ ⁺, is about 37:56:7. Note that, although FIG. 19 shows thedata obtained when the source gas is PH₃, the ratio between the hydrogenion species is about the same when a 100% hydrogen gas is used as asource gas, as well. In the case of the ion source from which the datashown in FIG. 19 is obtained, H₃ ⁺, of H⁺, H₂ ⁺, and H₃ ⁺, is generatedat a proportion of only about 7%. On the other hand, in the case of theion source from which the data shown in FIG. 18 is obtained, theproportion of H₃ ⁺ can be up to 50% or higher (under the aforementionedconditions, about 80%). This is thought to result from the pressure andelectric field inside a chamber, which is clearly shown in the aboveconsideration.

(H₃ ⁺ Irradiation Mechanism)

When a plasma that contains a plurality of ion species as shown in FIG.18 is generated and a single-crystalline semiconductor substrate isirradiated with the generated ion species without any mass separationbeing performed, the surface of the single-crystalline semiconductorsubstrate is irradiated with each of H⁺, H₂ ⁺, and H₃ ⁺ ions. In orderto reproduce the mechanism, from the irradiation with ions to theformation of an ion-introduced region, the following five types ofmodels are considered.

Model 1, where the ion species used for irradiation is H⁺, which isstill H⁺ (H) after the irradiation.

Model 2, where the ion species used for irradiation is H₂ ⁺, which isstill H₂ ⁺ (H₂) after the irradiation.

Model 3, where the ion species used for irradiation is H₂ ⁺, whichsplits into two H atoms (H⁺ ions) after the irradiation.

Model 4, where the ion species used for irradiation is H₃ ⁺, which isstill H₃ ⁺ (H₃) after the irradiation.

Model 5, where the ion species used for irradiation is H₃ ⁺, whichsplits into three H atoms (H⁺ ions) after the irradiation.

(Comparison of Simulation Results with Measured Values)

Based on the above models, the irradiation of an Si substrate withhydrogen ion species was simulated. As simulation software, SRIM, theStopping and Range of Ions in Matter (an improved version of TRIM, theTransport of Ions in Matter, which is simulation software for ionintroduction processes by a Monte Carlo method) was used. Note that, forthe calculation, a calculation based on Model 2 was performed with theH₂ ⁺ replaced by H⁺ that has twice the mass. In addition, a calculationbased on Model 4 was performed with the H₃ ⁺ replaced by H⁺ that hasthree times the mass. Furthermore, a calculation based on Model 3 wasperformed with the H₂ ⁺ replaced by H⁺ that has half the kinetic energy,and a calculation based on Model 5, with the H₃ ⁺ replaced by H⁺ thathas one-third the kinetic energy.

Note that SRIM is software intended for amorphous structures, but SRIMcan be applied to cases where irradiation with the hydrogen ion speciesis performed with high energy at a high dose. This is because thecrystal structure of an Si substrate changes into a non-single-crystalstructure due to the collision of the hydrogen ion species with Siatoms.

FIG. 20 shows the calculation results obtained when irradiation with thehydrogen ion species (irradiation with 100,000 atoms for H) is performedusing Models 1 to 5. FIG. 20 also shows the hydrogen concentration(secondary ion mass spectrometry (SIMS) data) in an Si substrateirradiated with the hydrogen ion species of FIG. 18. The results ofcalculations performed using Models 1 to 5 are expressed on the verticalaxis (right axis) as the number of hydrogen atoms, and the SIMS data isexpressed on the vertical axis (left axis) as the density of hydrogenatoms. The horizontal axis represents depth from the surface of an Sisubstrate. If the SIMS data, which is measured values, is compared withthe calculation results, Models 2 and 4 obviously do not match the peaksof the SIMS data and a peak corresponding to Model 3 cannot be observedin the SIMS data. This shows that the contribution of each of Models 2to 4 is relatively small. Considering that the kinetic energy of ions ison the order of kiloelectron volts whereas the H—H bond energy is onlyabout several electron volts, it is thought that the contribution ofeach of Models 2 and 4 is small because H₂ ⁺ and H₃ ⁺ mostly split intoH⁺ or H by colliding with Si atoms.

Accordingly, Models 2 to 4 will not be considered hereinafter. FIGS. 21to 23 each show the calculation results obtained when irradiation withthe hydrogen ion species (irradiation with 100,000 atoms for H) isperformed using Models 1 and 5. FIGS. 21 to 23 also each show thehydrogen concentration (SIMS data) in an Si substrate irradiated withthe hydrogen ion species of FIG. 18, and the simulation results fittedto the SIMS data (hereinafter referred to as a fitting function). Here,FIG. 21 shows the case where the accelerating voltage is 80 kV; FIG. 22,the case where the accelerating voltage is 60 kV; and FIG. 23, the casewhere the accelerating voltage is 40 kV. Note that the results ofcalculations performed using Models 1 and 5 are expressed on thevertical axis (right axis) as the number of hydrogen atoms, and the SIMSdata and the fitting function are expressed on the vertical axis (leftaxis) as the density of hydrogen atoms. The horizontal axis representsdepth from the surface of an Si substrate.

The fitting function is obtained using the calculation formula givenbelow, in consideration of Models 1 and 5. Note that, in the calculationformula, X and Y represent fitting parameters and V represents volume.

(Fitting Function)=X/V×(Data of Model 1)+Y/V×(Data of Model 5)

In consideration of the ratio between ion species used for actualirradiation (H⁺:H₂ ⁺:H₃ ⁺ is about 1:1:8), the contribution of H₂ ⁺(i.e., Model 3) should also be considered; however, Model 3 is excludedfrom the consideration given here for the following reasons:

Because the amount of hydrogen introduced through the irradiationprocess represented by Model 3 is lower than that introduced through theirradiation process of Model 5, there is no significant influence evenif Model 3 is excluded from the consideration (no peak appears in theSIMS data either).

Model 3, the peak position of which is close to that of Model 5, islikely to be obscured by channeling (movement of atoms due to crystallattice structure) that occurs in Model 5. That is, it is difficult toestimate fitting parameters for Model 3. This is because this simulationassumes amorphous Si and the influence due to crystallinity is notconsidered.

FIG. 24 lists the aforementioned fitting parameters. At any of theaccelerating voltages, the ratio of the amount of H introduced accordingto Model 1 to that introduced according to Model 5 is about 1:42 to 1:45(the amount of H in Model 5, when the amount of H in Model 1 is definedas 1, is about 42 to 45), and the ratio of the number of ions used forirradiation, H⁺ (Model 1) to that of H₃ ⁺ (Model 5) is about 1:14 to1:15 (the amount of H₃ ⁺ in Model 5, when the amount of H⁺ in Model 1 isdefined as 1, is about 14 to 15). Considering that Model 3 is notconsidered and the calculation assumes amorphous Si, it can be said thatvalues close to that of the ratio between ion species used for actualirradiation (H⁺:H₂ ⁺:H₃ ⁺ is about 1:1:8) is obtained.

(Effects of Use of H₃ ⁺)

A plurality of benefits resulting from H₃ ⁺ can be enjoyed byirradiation of a substrate with hydrogen ion species with a higherproportion of H₃ ⁺ as shown in FIG. 18. For example, because H₃ ⁺ splitsinto H⁺, H, or the like to be introduced into a substrate, ionintroduction efficiency can be improved compared with the case ofirradiation mainly with H⁺ or H₂ ⁺. This leads to an improvement insemiconductor substrate production efficiency. In addition, because thekinetic energy of H⁺ or H after H₃ ⁺ splits similarly tends to be low,H₃ ⁺ is suitable for manufacture of thin semiconductor layers.

Note that, in this specification, a method is described in which an iondoping apparatus that is capable of irradiation with the hydrogen ionspecies as shown in FIG. 18 is used in order to efficiently performirradiation with H₃ ⁺. Ion doping apparatuses are inexpensive andexcellent for use in large-area treatment. Therefore, by irradiationwith H₃ ⁺ by use of such an ion doping apparatus, significant effectssuch as an improvement in semiconductor characteristics, an increase inarea, a reduction in costs, and an improvement in production efficiencycan be obtained. On the other hand, if first priority is given toirradiation with H₃ ⁺, there is no need to interpret the presentinvention as being limited to the use of an ion irradiation apparatus.

Even when the ions are mass-separated and introduced to thesemiconductor substrate 106, the separation layer 107 can be formedsimilarly. In this case also, it is preferable that ions with a largemass (e.g., H₃ ⁺ ions) be selected and introduced to the semiconductorsubstrate since the effect similar to the above can be obtained.

Other than hydrogen, deuterium or an inert gas such as helium can alsobe selected as the gas from which ion species are generated. When heliumis used as a source gas and an ion doping apparatus which does not havea mass-separation function is used, an ion beam with a high proportionof He⁺ ions can be obtained. By introducing such ions to thesemiconductor substrate 106, microcavities can be formed and theseparation layer 107 similar to the above can be provided in thesemiconductor substrate 106.

Since introduction of ions needs to be conducted in high dose conditionsin formation of the separation layer, a surface of the semiconductorsubstrate 106 becomes rough in some cases. Therefore, a dense film maybe provided on the surface to which ions are to be introduced. Forexample, a silicon nitride film, a silicon nitride oxide film, or thelike having a thickness of 50 nm to 200 nm may be provided as aprotective film against ion introduction.

Next, a silicon oxide film is formed as the bonding layer 102 on thesurface on the side of forming a bond with a supporting substrate (seeFIG. 3B). The thickness of the silicon oxide film may be set at 10 nm to200 nm, preferably 10 nm to 100 nm, and more preferably 20 nm to 50 nm.As the silicon oxide film, a silicon oxide film formed by chemical vapordeposition using an organic silane gas in the above-described manner ispreferable. Another silicon oxide film formed by chemical vapordeposition using a silane gas can also be used. In the film formation bychemical vapor deposition, a film formation temperature of, for example,350° C. or lower is employed as such a temperature that does not causedegassing from the separation layer 107 formed in the single-crystallinesemiconductor substrate. In addition, heat treatment for separating thesingle-crystalline semiconductor layer from a single-crystalline orpolycrystalline semiconductor substrate is carried out at a heattreatment temperature higher than the film formation temperature.

The supporting substrate 100 and a surface of the bonding layer 102which is formed on the semiconductor substrate 106 are faced each otherand made in close contact, thereby forming a bond (FIG. 3C). Thesurfaces which form the bond are cleaned sufficiently. By making thesupporting substrate 100 and the surface of the bonding layer 102 faceeach other and pressing one part thereof from the outside, van der Waalsforce is increased due to reduction in distance between the surfaces andcontribution of hydrogen bonding, so that the supporting substrate 100and the surface of the bonding layer 102 are bonded. Further, since thefacing surfaces of the supporting substrate 100 and the bonding layer102 are made in close contact in a region near the pressed part, thebonded area spreads to the entire surfaces.

To form a favorable bond, one or both of surfaces that form a bond maybe activated. For example, a surface that forms a bond is irradiatedwith an atomic beam or an ion beam. In the case of utilizing the atomicbeam or the ion beam, an inert gas neutral atomic beam or an inert gasion beam of argon or the like can be used. Alternatively, plasmairradiation or radical treatment may be conducted. By such surfacetreatment, even at a temperature of 200° C. to 400° C., the bondingstrength between different kinds of materials can be increased.

First heat treatment is conducted with the semiconductor substrate 106and the supporting substrate 100 superposed on each other. By the firstheat treatment, the semiconductor substrate 106 is separated in thestate that a thin semiconductor layer (single-crystalline semiconductorlayer 101) remains over the supporting substrate 100 (FIG. 4A). Thefirst heat treatment is preferably conducted at a temperature equal toor higher than a film formation temperature of the bonding layer 102,and specifically, at a temperature equal to or higher than 400° C. andlower than 600° C. By conducting heat treatment at a temperature withinthis temperature range, the volume of microcavities formed in theseparation layer 107 is changed, so that a semiconductor substrate 106can be cleaved along the separation layer 107. Since the bonding layer102 and the supporting substrate 100 are bonded, the single-crystallinesemiconductor layer 101 having the same crystallinity as thesemiconductor substrate 106 is fixed on the supporting substrate 100. Bythis heat treatment, a covalent bond is generated from the hydrogenbond, and the bonding strength is increased.

Next, second heat treatment is conducted in the state that thesingle-crystalline semiconductor layer 101 is bonded to the supportingsubstrate 100 (FIG. 4B). The second heat treatment is preferablyconducted at a temperature which is higher than the temperature of thefirst heat treatment and does not exceed a strain point of thesupporting substrate 100. Alternatively, when the first heat treatmentand the second heat treatment are conducted at the same temperature, itis preferable that the treatment time of the second heat treatment beset to be longer. In the heat treatment, the supporting substrate 100and/or the single-crystalline semiconductor layer 101 may be heated bythermal conduction heating, convection heating, radiation heating, orthe like. As a heat treatment apparatus, an electrically-heated oven, alamp anneal furnace, or the like can be used. The second heat treatmentmay be conducted at multiple temperatures by changing the temperature.Further, a rapid thermal annealing (RTA) apparatus may also be used. Inthe case of conducting heat treatment with an RTA apparatus, the heattreatment can be conducted at a temperature close to a strain point of asubstrate or a little higher than the strain point.

By the second heat treatment, the residual stress in thesingle-crystalline semiconductor layer 101 can be relaxed. That is, bythe second heat treatment, thermal distortion generated due todifference in coefficient of expansion between the supporting substrate100 and the single-crystalline semiconductor layer 101 is relaxed. Thesecond heat treatment is also effective for recovering crystallinity ofthe single-crystalline semiconductor layer 101 whose crystallinity isdamaged by ion introduction. Further, the second heat treatment is alsoeffective for recovering damages of the single-crystalline semiconductorlayer 101, which have been caused in separation by the first heattreatment after bonding the semiconductor substrate 106 and thesupporting substrate 100. Furthermore, by the first heat treatment andthe second heat treatment, a hydrogen bond can be changed into astronger covalent bond. Since the heat capacity of a sample becomessmaller by separating the semiconductor substrate 106, the amount ofheat required for the second heat treatment is reduced. By the secondheat treatment, the semiconductor substrate 106 is separated from thesingle-crystalline semiconductor layer 101; accordingly, damages to thesurface of the single-crystalline semiconductor layer 101 can beprevented.

Chemical mechanical polishing (CMP) treatment may be conducted in orderto planarize the surface of the single-crystalline semiconductor layer101. The CMP treatment can be conducted after the first heat treatmentor the second heat treatment. In fact, if the CMP treatment is conductedbefore the second heat treatment, the surface of the single-crystallinesemiconductor layer 101 can be planarized and a damaged layer on thesurface generated by the CMP treatment can be restored by the secondheat treatment.

In either case, by combining the first heat treatment and the secondheat treatment as in this embodiment mode, a crystalline semiconductorlayer having excellent crystallinity can be provided over a supportingsubstrate that is weak to heat such as a glass substrate.

FIGS. 5A to 5C show a manufacturing process of a substrate with an SOIstructure, which has a single-crystalline semiconductor layer and inwhich a bonding layer is provided on the supporting substrate side.

Ions accelerated by electric field are introduced to a predetermineddepth of the semiconductor substrate 106 on which a silicon oxide layer105 is formed, and the separation layer 107 is formed (FIG. 5A). Theintroduction of the ions is conducted similarly to the case of FIG. 3A.By forming the silicon oxide layer 105 on the surface of thesemiconductor substrate 106, the surface can be prevented from beingdamaged by the introduction of the ions; therefore, loss of planaritycan be prevented.

The supporting substrate 100 over which the barrier layer 103 and thebonding layer 102 are formed and a surface of the silicon oxide layer105 provided on the semiconductor substrate 106 are made in closecontact with each other and a bond is formed (FIG. 5B). In this state,first heat treatment is conducted. The first heat treatment ispreferably conducted at a temperature equal to or higher than a filmformation temperature of the bonding layer 102, and specifically at atemperature equal to or higher than 400° C. and lower than 600° C. Bythis heat treatment, the volume of microcavities formed in theseparation layer 107 is changed, and the semiconductor substrate 106 canbe cleaved. The single-crystalline semiconductor layer 101 having thesame crystallinity as the semiconductor substrate 106 is formed over thesupporting substrate 100 (FIG. 5C).

Next, second heat treatment is conducted in the state where thesingle-crystalline semiconductor layer 101 is bonded to the supportingsubstrate 100. The second heat treatment is preferably conducted at atemperature which is higher than the temperature of the first heattreatment and does not exceed a strain point of the supporting substrate100. Alternatively, when the first heat treatment and the second heattreatment are conducted at the same temperature, it is preferable thatthe treatment time of the second heat treatment be set to be longer. Inthe heat treatment, the supporting substrate 100 and/or thesingle-crystalline semiconductor layer 101 may be heated by thermalconduction heating, convection heating, radiation heating, or the like.By the second heat treatment, the residual stress in thesingle-crystalline semiconductor layer 101 can be relaxed. The secondheat treatment is also effective in recovering damages of thesingle-crystalline semiconductor layer 101, which have been caused inseparation by the first heat treatment.

FIGS. 16A to 16C show another mode in the case where a bonding layer isprovided on the supporting substrate side and a single-crystallinesemiconductor layer is bonded. First, the separation layer 107 is formedin the semiconductor substrate 106 (FIG. 16A). The introduction withions for forming the separation layer 107 is conducted with an iondoping apparatus. In this step, the semiconductor substrate 106 isirradiated with plural kinds of ions with different masses which areaccelerated by high electric field. Since there is a possibility ofdamaging the planarity of the surface of the semiconductor substrate 106by irradiation with the ions, the silicon oxide layer 105 is preferablyprovided as a protective film. The silicon oxide layer 105 may be formedby thermal oxidation or employing a chemical oxide. The chemical oxidecan be formed by soaking the semiconductor substrate 106 in an oxidizingchemical solution. For example, a chemical oxide is formed on thesurface when the semiconductor substrate 106 is treated with anozone-containing aqueous solution. Alternatively, any of a siliconoxynitride film, silicon nitride oxide film, and a silicon oxide filmformed using TEOS, which are formed by a plasma CVD method, may be used.

The barrier layer 103 is preferably provided for the supportingsubstrate 100. By providing the barrier layer 103, thesingle-crystalline semiconductor layer 101 can be prevented from beingcontaminated by diffusion of a movable ion impurity such as an alkalimetal or an alkaline earth metal from the glass substrate used as thesupporting substrate 100. The barrier layer 103 has one layer or aplurality of layers. For example, a silicon nitride film or a siliconnitride oxide film, which is highly effective in blocking ions of sodiumor the like, is used as a first layer, and a silicon oxide film or asilicon oxynitride film is provided as a second layer thereon. The firstlayer is a dense insulating film to prevent diffusion of impurities,whereas the second layer is provided for relaxing stress so thatinternal stress of the first film does not affect an upper layer. Byproviding the barrier layer 103 for the supporting substrate 100, thesubstrate to be bonded to a single-crystalline semiconductor layer canbe selected from a wider range.

The semiconductor substrate 106 and the supporting substrate 100 overwhich the bonding layer 102 is provided as an upper layer of the barrierlayer 103 are bonded (FIG. 16B). The silicon oxide layer 105 provided asa protective film on the surface of the semiconductor substrate 106 isremoved with hydrofluoric acid, and the surface of the semiconductor isexposed. The outermost surface of the semiconductor substrate 106 ispreferable as long as it is terminated by hydrogen by treatment using ahydrofluoric acid solution. In formation of the bond, a hydrogen bond isformed by hydrogen termination of the surface, and a favorable bond canbe formed. Further, irradiation with ions of an inert gas may beconducted so that a dangling bond is exposed on the outermost surface ofthe semiconductor substrate 106, and a bond may be formed in vacuum.

In this state, first treatment is conducted. The first heat treatment ispreferably conducted at a temperature equal to or higher than a filmformation temperature of the bonding layer 102, and specifically at atemperature equal to or higher than 400° C. and lower than 600° C. Bythis heat treatment, the volume of microcavities formed in theseparation layer 107 is changed, and the semiconductor substrate 106 canbe cleaved. The single-crystalline semiconductor layer 101 having thesame crystallinity as the semiconductor substrate 106 is formed over thesupporting substrate 100 (FIG. 16C).

Next, second heat treatment is conducted in the state where thesingle-crystalline semiconductor layer 101 is bonded to the supportingsubstrate 100. The second heat treatment is preferably conducted at atemperature which is higher than the temperature of the first heattreatment and does not exceed a strain point of the supporting substrate100. Alternatively, when the first heat treatment and the second heattreatment are conducted at the same temperature, it is preferable thatthe treatment time of the second heat treatment be set to be longer. Inthe heat treatment, the supporting substrate 100 and/or thesingle-crystalline semiconductor layer 101 may be heated by thermalconduction heating, convection heating, radiation heating, or the like.By the second heat treatment, the residual stress in thesingle-crystalline semiconductor layer 101 can be relaxed. The secondheat treatment is also effective in recovering damages of thesingle-crystalline semiconductor layer 101, which have been caused inseparation by the first heat treatment.

According to this embodiment mode, even when the supporting substrate100 having an allowable temperature limit of 700° C. or lower such as aglass substrate is used, the single-crystalline semiconductor layer 101which can be strongly bonded in a bonding portion can be obtained. Asthe supporting substrate 100, a variety of glass substrates that areused in the electronics industry and that are called non-alkali glasssubstrates, such as aluminosilicate glass substrates,aluminoborosilicate glass substrates, and barium borosilicate glasssubstrates can be used. In other words, a single-crystallinesemiconductor layer can be formed over a substrate that is longer thanone meter on each side. With the use of such a large-area substrate, notonly a display device such as a liquid crystal display but also asemiconductor integrated circuit can be manufactured.

Next, a semiconductor device of this embodiment mode is described withreference to FIGS. 6A to 7. First, the single-crystalline semiconductorlayer 101 is provided over the supporting substrate 100 with the bondinglayer 102 interposed therebetween (FIG. 6A). The thickness of thesingle-crystalline semiconductor layer 101 is from 5 nm to 500 nm,preferably from 10 nm to 200 nm, and more preferably 10 nm to 60 nm. Thethickness of the single-crystalline semiconductor layer 101 can beappropriately set by controlling the depth of the separation layer 107as described with reference to FIGS. 3A to 3C. To control the thresholdvoltage, a p-type impurity element such as boron, aluminum, or galliumis added to the single-crystalline semiconductor layer 101. For example,boron may be added as a p-type impurity element at a concentration equalto or higher than 1×10¹⁶ cm⁻³ and equal to or lower than 1×10¹⁸ cm⁻³. Astack of a silicon nitride layer and a silicon oxide layer is formed asthe barrier layer 103 for the supporting substrate 100. By provision ofthe barrier layer for the supporting substrate 100, contamination of thesingle-crystalline semiconductor layer 101 can be prevented. Note thatinstead of the silicon nitride layer, a silicon nitride oxide layer, analuminum nitride layer, or an aluminum nitride oxide layer may be used.

The single-crystalline semiconductor layer 101 is etched and separatedinto island shapes in accordance with arrangement of semiconductorelements (FIG. 6B). After the island-shaped single-crystallinesemiconductor layers 101 are exposed, gate insulating layers 110, gateelectrodes 111, and sidewall insulating layers 112 are formed, and thenfirst impurity regions 113 and second impurity regions 114 are formed(FIG. 6C). An insulating layer 115 is formed using silicon nitride andused as a hard mask in etching the gate electrodes 111.

An interlayer insulating layer 116 is formed. The interlayer insulatinglayer 116 is formed by formation of a BPSG (boron phosphorus siliconglass) film or application of an organic resin typified by polyimide(FIG. 6D). Contact holes 117 are formed in the interlayer insulatinglayer 116. The contact holes 117 have a structure of self-alignedcontact formed by utilizing the sidewall insulating layers 112. Ofcourse, the contact holes 117 do not necessarily employ the self-alignedcontact.

Then, a wiring 119 is formed in accordance with the contact holes 117.The wiring 119 is formed using aluminum or an aluminum alloy, and metalfilms of molybdenum, chromium, titanium, or the like are formed asbarrier metals in an upper layer and a lower layer (FIG. 7).

In this manner, a field-effect transistor can be manufactured using thesingle-crystalline semiconductor layer 101 that is bonded to thesupporting substrate 100. Because the single-crystalline semiconductorlayer 101 according to this embodiment mode is a single-crystallinesemiconductor with uniform crystal orientation, a homogeneous,high-performance field-effect transistor can be obtained. In otherwords, it is possible to suppress inhomogeneity of values of importanttransistor characteristics, such as threshold voltage and mobility, andto achieve high performance such as high mobility.

FIG. 8 shows an example of a microprocessor 200 as an example of asemiconductor device. The microprocessor 200 is manufactured using thesemiconductor substrate of this embodiment mode as described above. Themicroprocessor 200 includes an arithmetic logic unit (ALU) 201, an ALUcontroller 202, an instruction decoder 203, an interrupt controller 204,a timing controller 205, a register 206, a register controller 207, abus interface (Bus I/F) 208, a read-only memory 209, and a ROM interface(ROM I/F) 210.

An instruction input to the microprocessor 200 through the bus interface208 is input to the instruction decoder 203, decoded therein, and theninput to the ALU controller 202, the interrupt controller 204, theregister controller 207, and the timing controller 205. The ALUcontroller 202, the interrupt controller 204, the register controller207, and the timing controller 205 conduct various controls based on thedecoded instruction. Specifically, the ALU controller 202 generatessignals for controlling the operation of the ALU 201. While themicroprocessor 200 is executing a program, the interrupt controller 204processes an interrupt request from an external input/output device or aperipheral circuit based on its priority or a mask state. The registercontroller 207 generates an address of the register 206, and reads andwrites data from and to the register 206 in accordance with the state ofthe microprocessor 200. The timing controller 205 generates signals forcontrolling timing of operation of the ALU 201, the ALU controller 202,the instruction decoder 203, the interrupt controller 204, and theregister controller 207. For example, the timing controller 205 isprovided with an internal clock generator for generating an internalclock signal CLK2 based on a reference clock signal CLK1, and suppliesthe clock signal CLK2 to the various above-mentioned circuits. Note thatthe microprocessor 200 shown in FIG. 8 is only an example in which theconfiguration is simplified, and an actual microprocessor may havevarious configurations depending on the uses.

The microprocessor 200 can achieve not only increase in processing speedbut also reduction in power consumption because an integrated circuit isformed using a single-crystalline semiconductor layer with uniformcrystal orientation which is bonded over a supporting substrate havingan insulating surface.

Next, an example of a semiconductor device having an arithmetic functionthat can transmit and receive data without contact is described withreference to FIG. 9. FIG. 9 shows an example of a computer that operatesto transmit and receive signals to and from an external device bywireless communication (such a computer is hereinafter referred to as an“RFCPU”). An RFCPU 211 includes an analog circuit portion 212 and adigital circuit portion 213. The analog circuit portion 212 includes aresonance circuit 214 with a resonance capacitor, a rectifier circuit215, a constant voltage circuit 216, a reset circuit 217, an oscillationcircuit 218, a demodulation circuit 219, and a modulation circuit 220.The digital circuit portion 213 includes an RF interface 221, a controlregister 222, a clock controller 223, an interface 224, a centralprocessing unit 225, a random-access memory 226, and a read-only memory227.

The operation of the RFCPU 211 having such a configuration is asfollows. The resonance circuit 214 generates an induced electromotiveforce based on a signal received by an antenna 228. The inducedelectromotive force is stored in a capacitor portion 229 through therectifier circuit 215. This capacitor portion 229 is preferably formedusing a capacitor such as a ceramic capacitor or an electric doublelayer capacitor. The capacitor portion 229 does not need to beintegrated with the RFCPU 211 and it is acceptable as long as thecapacitor portion 229 is mounted as a different component on a substratehaving an insulating surface which is included in the RFCPU 211.

The reset circuit 217 generates a signal that resets the digital circuitportion 213 to be initialized. For example, the reset circuit 217generates, as a reset signal, a signal that rises with delay afterincrease in the power supply voltage. The oscillation circuit 218changes the frequency and the duty ratio of a clock signal in accordancewith a control signal generated by the constant voltage circuit 216. Thedemodulation circuit 219 having a low pass filter, for example,binarizes amplitude fluctuation of reception signals of an amplitudeshift keying (ASK) system. The modulation circuit 220 transmitstransmission data by changing the amplitude of transmission signals ofan amplitude shift keying (ASK) system. The modulation circuit 220changes the resonance point of the resonance circuit 214, therebychanging the amplitude of communication signals. The clock controller223 generates a control signal for changing the frequency and the dutyratio of the clock signal in accordance with the power supply voltage orcurrent consumption in the central processing unit 225. The power supplyvoltage is monitored by a power supply control circuit 230.

A signal input from the antenna 228 to the RFCPU 211 is demodulated bythe demodulation circuit 219 and then decomposed into a control command,data, and the like by the RF interface 221. The control command isstored in the control register 222. The control command includes readingof data stored in the read-only memory 227, writing of data to therandom-access memory 226, an arithmetic instruction to the centralprocessing unit 225, and the like. The central processing unit 225accesses the read-only memory 227, the random-access memory 226, and thecontrol register 222 via the interface 224. The interface 224 has afunction of generating an access signal for any of the read-only memory227, the random-access memory 226, and the control register 222 based onan address the central processing unit 225 requests.

As an arithmetic method of the central processing unit 225, a method maybe employed in which the read-only memory 227 stores an operating system(OS) and a program is read and executed at the time of startingoperation. Alternatively, a method may be employed in which a circuitdedicated to arithmetic is formed as an arithmetic circuit andarithmetic processing is conducted using hardware. In a method in whichboth hardware and software are used, part of processing can be conductedby a circuit dedicated to arithmetic and the other part of thearithmetic processing can be conducted by the central processing unit225 using a program.

The RFCPU 211 can achieve not only increase in processing speed but alsoreduction in power consumption because an integrated circuit is formedusing a single-crystalline semiconductor layer with uniform crystalorientation which is bonded over a substrate having an insulatingsurface or an insulating substrate. This makes it possible to ensure theoperation for a long period even when the capacitor portion 229 whichsupplies power is downsized. Although FIG. 9 shows a mode of the RFCPU,the semiconductor device may be an IC tag or the like as long as it hasa communication function, an arithmetic processing function, and amemory function.

The single-crystalline semiconductor layer 101 exemplified in FIGS. 1Ato 2B can be bonded to a large glass substrate called a mother glasswith which a display panel can be manufactured. FIG. 10 shows the casewhere the single-crystalline semiconductor layer 101 is bonded to amother glass that is used as the supporting substrate 100. A pluralityof display panels are taken from a mother glass, and thesingle-crystalline semiconductor layers 101 are preferably bonded so asto match formation regions of display panels 122. Since a mother glasssubstrate has a larger area than a semiconductor substrate, it ispreferable that the single-crystalline semiconductor layers 101 bearranged separately as shown in FIG. 10. The display panels 122 eachincludes a scanning line driver circuit region 123, a signal line drivercircuit region 124, and a pixel formation region 125. Thesingle-crystalline semiconductor layers 101 are bonded to the supportingsubstrate 100 (mother glass) so that these regions are included.

FIGS. 11A and 11B show an example of a pixel of a liquid crystal displaypanel in which a pixel transistor is formed using the single-crystallinesemiconductor layer 101. FIG. 11A is a plane view of a pixel, in which ascanning line 126 intersects with the single-crystalline semiconductorlayer 101 and the single-crystalline semiconductor layer 101 isconnected to a signal line 127 and a pixel electrode 128. FIG. 11B is across-sectional view corresponding to a line J-K in FIG. 11A.

In FIG. 11B, a stack of a silicon nitride layer and a silicon oxidelayer is formed as the barrier layer 103 over the supporting substrate100. The single-crystalline semiconductor layer 101 is bonded to thebarrier layer 103 with the use of the bonding layer 102. The pixelelectrode 128 is provided over an insulating layer 118. By etching ofthe insulating layer 118, a step in the form of a depression isgenerated in a contact hole, by which the single-crystallinesemiconductor layer 101 is connected to the signal line 127, and acolumnar spacer 131 is provided so as to fill the step. A countersubstrate 129 is provided with a counter electrode 130. A liquid crystallayer 132 is formed in a space which is formed by the columnar spacer131.

FIG. 12A shows an example of an electroluminescent display panel inwhich a transistor in a pixel portion is formed using thesingle-crystalline semiconductor layer 101. FIG. 12A is a plane view ofthe pixel, and a selection transistor 133 connected to the signal line127 and a display control transistor 134 connected to a current supplyline 135 are included. This display panel has a structure where alight-emitting element in which a layer (EL layer) formed including anelectroluminescent material is sandwiched between electrodes is providedin each pixel. The pixel electrode 128 is connected to the displaycontrol transistor 134. FIG. 12B is a cross-sectional view showing amain section of the pixel.

In FIG. 12B, the structure of the supporting substrate 100, the barrierlayer 103, the bonding layer 102, the single-crystalline semiconductorlayer 101, the insulating layer 118, and the like is similar to that ofFIG. 11B. The periphery of the pixel electrode 128 is surrounded by aninsulating partition wall layer 136. An EL layer 137 is formed over thepixel electrode 128. The counter electrode 130 is formed over the ELlayer 137. A pixel portion is filled with a filling resin 138, and thecounter substrate 129 is provided as a reinforcing board.

The display screen of the electroluminescent display panel of thisembodiment mode is formed by arranging such pixels in matrix. In thiscase, since channel portions of the transistors in the pixels are formedof the single-crystalline semiconductor layers 101, variation incharacteristics between the transistors does not arise; accordingly,there is an advantage in that there is no variation in light emissionluminance between pixels. Therefore, it is easier to drive alight-emitting element and control the luminance of the light-emittingelement by current, and a correction circuit which corrects variation intransistor characteristics becomes unnecessary, which can reduce theload on the driving circuit.

As described above, a single-crystalline semiconductor layer can beformed even over a mother glass with which a display panel ismanufactured to form a transistor. The transistor formed using asingle-crystalline semiconductor layer is superior to an amorphoussilicon transistor in all operating characteristics such as capacity ofcurrent drive; therefore, the transistor can be downsized. Accordingly,an aperture ratio of a pixel portion in the display panel can beimproved. Further, since a microprocessor like the one illustrated inFIGS. 8 and 9 can be formed, a function as a computer can be provided inthe display panel. Furthermore, a display in which data can be input andoutput without contact can be manufactured.

EMBODIMENT 1

Embodiment 1 will describe a manufacturing method of a substrate havingan SOI structure with reference to FIGS. 13A to 14B.

First, a silicon oxynitride film 305 is formed with a thickness of 100nm by a plasma CVD method using a SiH₄ gas and a N₂O gas over asingle-crystalline silicon substrate 301 from which a natural oxide filmis removed. In addition, a silicon nitride oxide film 306 is formed witha thickness of 50 nm using a SiH₄ gas, a N₂O gas, and a NH₃ gas (FIG.13A).

Then, hydrogen ions are introduced from a surface of the silicon nitrideoxide film 306 with an ion doping apparatus (FIG. 13B). In thisembodiment, hydrogen is ionized, and a separation layer 303 is formed inthe single-crystalline silicon substrate 301. Ion doping is conductedwith an accelerating voltage of 80 kV and a dose of 2×10¹⁶ ions/cm².

The ion doping apparatus has a system in which an ionized gas is notmass-separated and directly accelerated by electric field to beintroduced to a substrate. The ion doping apparatus includes a dopingchamber which is connected to an ion source and has a structure in whichthe doping chamber is exhausted with a vacuum pump to have a reducedpressure and a substrate positioned in the doping chamber is irradiatedwith ions. The ion source includes a plasma chamber and an accelerationelectrode system which withdraws ions produced in the plasma chamber. Asa plasma generation method in the ion source, a DC filament method ispreferably used. When the DC filament method is used, ions of pluralkinds of the same type of atom which have different masses can begenerated.

For example, in the case of performing ion doping by introducing ahydrogen gas, ion species of H⁺, H₂ ⁺, and H₃ ⁺ can be generated. Theion species of H⁺, H₂ ⁺, and H₃ ⁺ exist at proportions of 12%, 8%, and80%, respectively, and the separation layer 303 is formed byintroduction of the ion species to the single-crystalline siliconsubstrate 301. By letting higher-order ions having small massescontained in the separation layer 303 in this manner, cleavage of asingle-crystalline silicon substrate 301 can be easily formed in a heattreatment process. In this case, by providing the silicon nitride oxidefilm 306 and the silicon oxynitride film 305 over a surface to besubjected to ion doping of the single-crystalline silicon substrate 301,surface roughness of the single-crystalline silicon substrate 301 due toion doping can be prevented.

A silicon oxide film 304 is formed as a bonding layer over the siliconnitride oxide film 306 (FIG. 13C). The silicon oxide film 304 is formedwith a thickness of 50 nm by a plasma CVD method using tetraethoxysilane(TEOS: chemical formula: Si(OC₂H₅)₄) and an oxygen gas. The filmformation temperature is made at 350° C. or lower so that hydrogen isnot disorbed from the separation layer 303.

A glass substrate 300 subjected to ultrasonic cleaning and cleaning withozone-containing water and the single-crystalline silicon substrate 301are superposed on each other and pressed, with the silicon oxide film304 interposed therebetween, so that a bond is formed (FIG. 14A). Afterthat, first heat treatment is conducted to separate thesingle-crystalline silicon substrate 301 and the glass substrate 300that is a supporting substrate from each other (FIG. 14B). Then, secondheat treatment is conducted in a state that a single-crystallinesemiconductor layer 302 is bonded to the glass substrate 300.

In this embodiment, the first heat treatment was conducted for two caseswhere the heat treatment temperatures were 410° C. and 500° C. The heattreatment time for each case was two hours. Note that for each case ofthe heat treatment, preheating at 410° C. for 10 minutes was conductedbefore heating at the predetermined temperature, and slow cooling wasconducted at 410° C. for 2 hours after each case of the heat treatment.In addition, the second heat treatment was conducted at 550° C. for 2hours. For the second heat treatment, time for preheating and slowcooling is provided before and after the heat treatment, similarly tothe first heat treatment.

Table 1 shows the evaluation results of Raman spectroscopiccharacteristics for single-crystalline semiconductor layers providedover glass substrates, to each of which the first heat treatment and thesecond heat treatment are conducted. FIG. 15 shows Raman spectroscopicspectra. The evaluation of the Raman spectroscopic characteristics wascarried out with a laser Raman spectrometer (HORIBA U1000).

TABLE I Sample 1 Sample 2 Bulk Si First heat treatment 410° C. 500° C. —temperature Second heat treatment 550° C. 550° C. — temperature Ramanshift [cm⁻¹] 520.2 520.1 520.7 Raman intensity [cts/s] 905 1031 4384Full width at half 4.10 3.95 2.77 maximum [cm⁻¹]

As a comparative example, samples were manufactured with the samecondition of the first heat treatment and without the second heattreatment, and the Raman spectroscopic characteristics of the sampleswere evaluated similarly. The results are shown in Table 2.

TABLE 2 Comparative Comparative sample A sample B First heat treatment410° C. 500° C. temperature Second heat treatment — — temperature Ramanshift [cm⁻¹] 520.0 520.0 Raman intensity [cts/s] 582 747 Full width athalf 5.59 4.77 maximum [cm⁻¹]

When the samples of this embodiment and the samples of the comparativeexample are compared with reference to Tables 1 and 2 and FIG. 15, thefull widths at half-maximum of the Raman spectra of the samples of thisembodiment, to each of which the second heat treatment was conducted,are smaller. This shows that crystal defects and crystal distortion arereduced by the second heat treatment, thereby improving crystallinity.Further, the samples of this embodiment, to each of which the secondheat treatment is conducted, have stronger Raman intensities. The Ramanintensity is a comparative value, but the value tends to be higher asthe crystallinity is higher. Therefore, it is apparent that the secondheat treatment is effective for crystallinity recovery of asingle-crystalline semiconductor layer bonded to a glass substrate.

As described above, according to this embodiment, the single-crystallinesemiconductor layer 302 having relaxed distortion and superiorcrystallinity can be bonded to the glass substrate 300. Each of thesingle-crystalline semiconductor layers 302 manufactured in thisembodiment is strongly bonded to the glass substrate 300 and is notseparated therefrom even when a separation test using a tape isconducted. In other words, the single-crystalline semiconductor layercan be provided over a variety of glass substrates that are used in theelectronics industry and that are called non-alkali glass substrates,such as aluminosilicate glass substrates, aluminoborosilicate glasssubstrates, and barium borosilicate glass substrates. Various integratedcircuits and display devices can be manufactured using a substrate thatis longer than one meter on each side.

This application is based on Japanese Patent Application serial no.2007-112432 filed with Japan Patent Office on Apr. 20, 2007, the entirecontents of which are hereby incorporated by reference.

1. A manufacturing method of an SOI substrate comprising the steps of:introducing ions into a portion of a semiconductor substrate to form aseparation layer having a porous structure; forming an insulating filmover the semiconductor substrate; bonding the semiconductor substrateand a second substrate with the insulating film interposed therebetween;performing a first heat treatment by which the semiconductor substrateis separated at the separation layer so that a part of the semiconductorsubstrate remains as a semiconductor layer, over the second substrate;and performing a second heat treatment to the second substrate to whichthe semiconductor layer is bonded, wherein a temperature of the secondheat treatment is equal to or higher than that of the first heattreatment.
 2. The manufacturing method of an SOI substrate according toclaim 1, wherein the ions are H⁺, H₂ ⁺, and H₃ ⁺ ions.
 3. Themanufacturing method of an SOI substrate according to claim 2, wherein aproportion of the H₃ ⁺ ions is made higher than those of the other ionsamong the ions.
 4. The manufacturing method of an SOI substrateaccording to claim 1, wherein the temperature of the first heattreatment is equal to or higher than 400° C. and lower than 600° C., andwherein the temperature of the second heat treatment is equal to orlower than a strain point of the second substrate.
 5. The manufacturingmethod of an SOI substrate according to claim 1, wherein a treatmenttime of the second heat treatment is longer than that of the first heattreatment.
 6. The manufacturing method of an SOI substrate according toclaim 1, wherein the insulating film is a single layer of a film or astacked layer of a plurality of films selected from the group consistingof a silicon oxide film, a silicon nitride film, a silicon nitride oxidefilm, and a silicon oxynitride film.
 7. The SOI substrate according toclaim 1, wherein the second substrate is a glass substrate.
 8. Themanufacturing method of an SOI substrate according to claim 1, whereinthe insulating film is formed by chemical vapor deposition using anorganic silane gas.
 9. The manufacturing method of an SOI substrateaccording to claim 8, wherein the organic silane gas includes at leastone of tetraethoxysilane, tetramethylsilane,tetramethylcyclotetrasiloxane, octamethylcyclotetrasiloxane,hexamethyldisilazane, triethoxysilane, and trisdimethylaminosilane. 10.A manufacturing method of an SOI substrate comprising the steps of:introducing ions of plural kinds of the same type of atom which havedifferent masses into a portion of a semiconductor substrate to form aseparation layer having a porous structure in the semiconductorsubstrate; forming an insulating film over the semiconductor substrate;bonding the semiconductor substrate and a second substrate with theinsulating film interposed therebetween; performing a first heattreatment by which the semiconductor substrate is separated at theseparation layer so that a part of the semiconductor substrate remainsas a semiconductor layer, over the second substrate; and performing asecond heat treatment to the second substrate to which the semiconductorlayer is bonded, wherein a temperature of the second heat treatment isequal to or higher than that of the first heat treatment.
 11. Themanufacturing method of an SOI substrate according to claim 10, whereinthe ions of plural kinds of the same type of atom which have differentmasses are H⁺, H₂ ⁺, and H₃ ⁺ ions.
 12. The manufacturing method of anSOI substrate according to claim 11, wherein a proportion of the H₃ ⁺ions is made higher than those of the other ions among the ions.
 13. Themanufacturing method of an SOI substrate according to claim 10, whereinthe temperature of the first heat treatment is equal to or higher than400° C. and lower than 600° C., and wherein the temperature of thesecond heat treatment is equal to or lower than a strain point of thesecond substrate.
 14. The manufacturing method of an SOI substrateaccording to claim 10, wherein a treatment time of the second heattreatment is longer than that of the first heat treatment.
 15. Themanufacturing method of an SOI substrate according to claim 10, whereinthe insulating film is a single layer of a film or a stacked layer of aplurality of films selected from the group consisting of a silicon oxidefilm, a silicon nitride film, a silicon nitride oxide film, and asilicon oxynitride film.
 16. The SOI substrate according to claim 10,wherein the second substrate is a glass substrate.
 17. A manufacturingmethod of an SOI substrate comprising the steps of: introducing ionscomprising an H₃ ⁺ ion into a portion of a semiconductor substrate toform a separation layer having a porous structure in the semiconductorsubstrate; forming an insulating film over the semiconductor substrate;bonding the semiconductor substrate and a second substrate with theinsulating film interposed therebetween; performing a first heattreatment by which the semiconductor substrate is separated at theseparation layer so that a part of the semiconductor substrate remainsas a semiconductor layer, over the second substrate; and performing asecond heat treatment to the second substrate to which the semiconductorlayer is bonded, wherein a temperature of the second heat treatment isequal to or higher than that of the first heat treatment.
 18. Themanufacturing method of an SOI substrate according to claim 17, whereinthe ions further comprising H⁺ and H₂ ⁺ ions, and wherein a proportionof the H₃ ⁺ ions is made higher than those of the other ions among theions.
 19. The manufacturing method of an SOI substrate according toclaim 17, wherein the temperature of the first heat treatment is equalto or higher than 400° C. and lower than 600° C., and wherein thetemperature of the second heat treatment is equal to or lower than astrain point of the second substrate.
 20. The manufacturing method of anSOI substrate according to claim 17, wherein a treatment time of thesecond heat treatment is longer than that of the first heat treatment.21. The manufacturing method of an SOI substrate according to claim 17,wherein the insulating film is a single layer of a film or a stackedlayer of a plurality of films selected from the group consisting of asilicon oxide film, a silicon nitride film, a silicon nitride oxidefilm, and a silicon oxynitride film.
 22. The SOI substrate according toclaim 17, wherein the second substrate is a glass substrate.